The invention relates enhanced Agrobacterium transformation frequencies of plants due to overexpression of the histone H2A gene encoded by the Arabidopsis RAT5 gene. Agrobacterium tumefaciens is a gram negative soil bacterium that has been exploited by plant biologists to introduce foreign DNA into plants. However, there are some limitations on the use of this transforming vector, e.g. difficulties in transforming monocots, and transforming frequencies may be too low to be useful. Although known for this practical application, the actual mechanism of DNA transfer from bacteria to plants is not completely understood.
Agrobacterium tumefaciens genetically transforms plant cells by transferring a portion of the bacterial Ti-plasmid, designated the T-DNA, to the plant, and integrating the T-DNA into the plant genome. Little is known about the T-DNA integration process, and no plant genes involved in integration have previously been identified. The DNA that is transferred from Agrobacterium to the plant cell is a segment of the Ti, or tumor inducing, plasmid called the T-DNA (transferred DNA). Virulence (vir) genes responsible for T-DNA processing and transfer are reported to lie elsewhere on the Ti plasmid. The role of vir genes in T-DNA processing, the formation of bacterial channels for export of T-DNA, and the attachment of bacteria to the plant cell are reported (Sheng and Citovsky, 1996; Zupan and Zambryski, 1997). In contrast, little is known about the role of plant factors in T-DNA transfer and integration. The isolation of a putative plant factor has recently been reported. Ballas and Citovsky showed that a plant karyopherin xcex1 (AtKAP xcex1) can interact with VirD2 nuclear localization sequences in a yeast two-hybrid interaction system, and is presumably involved in nuclear translocation of the T-complex. Using a similar approach, a tomato type 2C protein phosphatase, DIG3, that can interact with the VirD2 NLS was identified. Unlike AtKAP xcex1, DIG3 plays a negative role in nuclear import. After the T-DNA/T-complex enters the nucleus, it must integrate into the plant chromosome. Plant chromosomal DNA is packaged into nucleosomes consisting primarily of histone proteins. The incoming T-DNA may have to interact with this nucleosome structure during the integration process. However, T-DNA may preferentially integrate into transcribed regions of the genome. These regions are believed to be temporarily free of histones. How exactly T-DNA integration takes place is unknown. Recent reports have implicated involvement of VirD2 protein in the T-DNA integration process. Plant proteins are also likely to be involved in this process (Deng et al., 1998; Ballas and Citovsky, 1997; Tao, et al.). Other evidence for the involvement of plant factors in T-DNA transfer and integration comes from identification of several ecotypes of Arabidopsis that are resistant to Agrobacterium transformation.
To identify plant genes involved in Agrobacterium-mediated transformation, a T-DNA tagged Arabidopsis library was screened for mutants that are resistant to Agrobacterium transformation (rat mutants). There are several steps in which plant genes are likely involved in the Agrobacterium-mediated transformation process. First, plant-encoded factors could be involved in the initial step of bacterial attachment to the plant cell surface. Mutants and ecotypes that are deficient in bacterial attachment have been identified and genes involved in bacterial attachment are currently being characterized. The next step in which a plant factor(s) could be involved is the transfer of T-strands from the bacteria to plant cells across the plant cell wall and membrane. After T-DNA/T-complex enters the cytoplasm of the plant cell, plant factors are required to transport the T-complex to the nucleus.
An Arabidopsis T-DNA tagged mutant, rat5, was characterized that is deficient in T-DNA integration and is resistant to Agrobacterium-mediated root transformation. Both genetic and DNA blot analyses indicated that there are two copies of T-DNA integrated as a tandem repeat at a single locus in rat5. No major rearrangements are in the rat5 plant DNA immediately surrounding the T-DNA insertion site. These data strongly suggest that in rat5 the T-DNA had inserted into a gene necessary for Agrobacterium-mediated transformation. The sequence of the T-DNA left border-plant junction indicated that the T-DNA had inserted into the 3xe2x80x2 untranslated region of a histone H2A gene. This insertion is upstream of the consensus polyadenylation signal. By screening an Arabidopsis ecotype Ws cDNA library and sequencing 20 different histone H2A cDNA clones, and by performing a computer data base search, at least six different histone H2A genes were shown. These genes encode proteins that are greater than 90% identical at the amino acid sequence level. Thus, the histone H2A genes comprise a small multi-gene family in Arabidopsis.
T-DNA integration does not appear to take place by homologous recombination, believed to be the most common method of foreign DNA integration in prokaryotes and lower eukaryotes, because no extensive homology between the T-DNA and target sequences has been found. T-DNA is reported to integrate by illegitimate recombination (Matsumoto el al., 1990; Gheysen et al., 1991; Mayerhofer et al., 1991; Ohba et al., 1995). Illegitimate recombination is the predominant mechanism of DNA integration into the genomes of higher plants (Britt, 1996; Offringa et al., 1990; Paszkowski et al., 1988).
Information on factors affecting Agrobacterium transformation frequencies in plants is needed to improve performance of this method.
The invention relates to increased Agrobacterium transformation frequencies in plants due to addition of at least one gene involved in host T-DNA integration, to the host plant. In an embodiment, addition of at least one histone H2A gene encoded by the Arabidopsis RAT5 gene enhances transformation frequencies, most likely due to overexpressing of histone as compound to the host""s natural expression levels. The gene can be either in transgenic plants or carried by the transforming agent, T-DNA for practice of the invention.
Overexpression of histone genes of the present invention overcomes the poor performance that limits the use of Agrobacterium as a transforming vector. Many plants can be transformed transiently by Agrobacterium so they express the transforming gene for a period of time, but are not stably transformed because of T-DNA integration problems. Therefore, transgenic plants are not produced. The gene H2A (RAT5) plays an important role in illegitimate recombination of T-DNA into the plant genome and the gene""s overexpression enhances transformation.
Transient and stable GUS (xcex2-glucuronidase) expression data and the assessment of the amount of T-DNA integrated into the genomes of wild-type and rat5 Arabidopsis plants indicated that the rat5 mutant is deficient in T-DNA integration needed for transformation. Complementing the rat5 mutation was accomplished by expressing the wild-type RAT5 histone H2A gene in the mutant plant. Surprisingly, overexpression of RAT5 in wild-type plants increased Agrobacterium transformation efficiency. Furthermore, transient expression of a RAT5 gene from the incoming T-DNA was sufficient to complement the rat5 mutant and to increase the transformation efficiency of wild-type Arabidopsis plants. The present invention provides methods and compositions to increase stable transformation frequency in plants using direct involvement of a plant histone gene in T-DNA integration.